The present invention relates in general to voltage controlled oscillators, and more particularly, to voltage controlled oscillators having a constant high Q over a wide tuning range.
Voltage controlled oscillators (VCOs) operated at the intermediate frequency (IF) of a television receiver are commonly used for synchronously demodulating audio and video information contained in the incoming radio frequency (RF) carrier. Such a VCO must be stable and must have a high Q and low phase noise. If the receiver is intended for worldwide use the VCO must also be tunable over a frequency range because each country has its own IF and demodulation standards. The variation in IF standards in effect around the world ranges from at least 34 megahertz to 56 megahertz. In the United States the standard television IF is 45.75 megahertz.
The IF oscillator requires a narrow bandwidth for good television channel selection and noise immunity. To achieve the narrow bandwidth tank circuits formed with an inductor and a capacitor are generally used because they have an inherently high Q and narrow bandwidth. However, tank circuits resonate at a fixed frequency so that additional circuit components are needed for frequency tuning. One known tuning method uses a tank circuit along with a capacitive device such as a varactor diode for varying the frequency. A control voltage is applied to the varactor diode to change its capacitance, thereby changing the resonant frequency. A disadvantage of using varactor diodes is that a large variation in control voltage is needed for tuning them, making them impractical to integrate on a semiconductor die with the VCO.
Another known tuning method uses a balanced variable reactance circuit to generate reactance currents in the tank circuit in response to a tuning signal. A balanced variable reactance circuit is easily integrated on a semiconductor die but has a disadvantage of introducing error currents into the tank circuit which result in wide variations in Q as the VCO is tuned across a frequency band. The result is nonuniform noise performance of certain television channels. Performance is even less uniform if inexpensive inductors with high series resistance are used in the tank circuit.
There is a need for a VCO in a television receiver which can be tuned over a wide range of IF frequencies and be manufactured at low cost. It would be a benefit if the VCO used a method for tuning which did not require using large control voltages and could be readily integrated on a semiconductor die. It would be a further benefit if a constant Q could be maintained across the tuning range so that the VCO had both low noise and stable operation. It would be even more advantageous if the method compensated for using inexpensive tank inductors having high resistance without lowering performance in the VCO.